Energy changes in a system

• Kinetic energy equation

  1/2 x Mass x (Velocity)²
• units

  • Energy (J)
  • Mass (kg)
  • Velocity (m/s)
• Equation for elastic potential energy

  1/2 x spring constant x (Extension)^2
• Units

  Energy (J)

  spring constant (N/m)

  Extension (m)
• Equation for gravitational potential energy

  Mass x gravitational field strength x height
• Units

  Energy (J)

  mass (Kg)

  Gravitational field strength (N/Kg)

  height (m)
• Define the specific heat capacity of a substance

  The amount of energy needed to raise the temperature of 1Kg of a substance by 1 degree
• Units for specific heat capacity

  Joules

  degree celsius

  kilogram
• What’s the definition of power

  The rate at which energy is transferred (or rate at which work is done)
• State two equations for power

  Power= energy transferred/time

  power=work done/time
• Units

  Energy (j)

  work done (j)

  time (s)
• What’s the u it of power

  Watts (w)
• Two motors lift the same mass through the same height. Motor A does this is half the time of motor B. Which dissipates the most power?

  Motor A

  The energy transferred is the same but the time taken is less
• Describe the energy changes involved when a ball is thrown upwards and then returns to its starting position. Ignore air resistance

  • Upwards: KE is converted to GPE
  • peak: maximum GPE, zero KE
  • downwards:GPE is converted to KE
• Describe the energy transfer for a bungee jumper

  • When falling, the GPE is converted to KE of jumper
  • as the cord tightens KE is converted and stored as elastic potential energy
  • at lowest point the jumpers initial GPE equals the EPE stored in the cord
• Explain why a bungee jumper slows down once the cord begins to stretch

  • Kinetic energy decreases since it is converted to elastic potential energy
  • since KE is proportional to (velocity)^2 as KE decreases so does velocity
• Give example of chemical energy stores

  • Food
  • fuel (wood,coal,petrol)
  • batteries
• 4 different stores of energy

  • Kinetic energy
  • gravitational potential energy
  • elastic potential energy
  • chemical energy